1) Field of the Invention
The present invention relates to a multilayer gas sensing element applicable to an exhaust system of an internal combustion engine of a vehicle or the like, and employable for the purpose of the detection of NOx concentration or the like, and further to a gas sensing element to be built in a gas sensor for combustion control of an internal combustion engine of a vehicle and other applications.
2) Description of the Related Art
Exhaust gases emitted from internal combustion engines of vehicles and others create air contamination, which causes serious problems in the modern society, and the purification standard regulations regarding NOx and others in the exhaust gases, which form pollution substances, have taken on an intensity year by year.
Meanwhile, it is considered that, if a concentration of NOx in an exhaust gas is detected and the detection result is feedbacked to an engine combustion control monitor, a catalyst monitor or the like, the exhaust gas purification is more efficiently achievable. Such a situation requires a gas sensing element capable of accurately detecting the NOx concentration in the exhaust gas.
As a well-known conventional multilayer gas sensing element, there is an element shown in FIG. 26.
In FIG. 26, in the multilayer gas sensing element, generally designated at reference numeral 9, an oxygen pump cell 92 is located to confront a first measured gas chamber 11 and, upon receipt of a voltage, pumps oxygen existing in the interior of the first measured gas chamber 11 into the exterior of the sensing element, or pumps oxygen from the exterior of the sensing element into the interior of the first measured gas chamber 11.
In addition, an oxygen monitor cell 93 capable of detecting an oxygen concentration in the interior of the first measured gas chamber 11 is provided to feedback-control the oxygen pump cell 92 so that the oxygen concentration in the first measured gas chamber 11, detected by the oxygen monitor cell 93, becomes a steady state.
Still additionally, a sensor cell 94 is provided in a second measured gas chamber 12 communicating with the first measured gas chamber 11, with the sensor cell 94 being designed to measure a concentration of NOx by measuring an ionic current occurring due to oxygen ions generated through the decomposition of NOx on an electrode.
Since the oxygen concentration in the first measured gas chamber 11 is controlled to become a steady state as mentioned above, the oxygen concentration in the second measured gas chamber 12 becomes constant. Therefore, the quantity of oxygen ions moving between the electrodes in the sensor cell 94, that is, the magnitude of the oxygen ionic current in the sensor cell 94, corresponds to a concentration of NOx.
Thus, the NOx concentration of a measured gas introduced into the first and second measured gas chambers 11 and 12 can be measured with high accuracy irrespective of an increase/decrease in oxygen concentration in the atmosphere outside the sensing element.
Meanwhile, in the multilayer gas sensing element, not until the cells including the oxygen pump cell, the sensor cell and the monitor cell are heated up to an activating temperature, they can exhibits their functions.
For the heating thereof, as shown in FIG. 26, the multilayer gas sensing element 9 is equipped integrally with a heater 19 including a heat generator 191 for generating heat in response to current supply and a heater lead 192.
In this heater 19, in a case in which the difference in electrical resistance value between the heat generator 191 and the heater lead 192 is small, the heater lead 192 generates head up to a temperature close to that of the heat generator 109. For this reason, the temperature in the vicinity of the heater lead 192 rises and the electrical resistance value thereof decreases to create a situation in which a leakage current tends to flow.
In addition, in the multilayer gas sensing element, the current flowing through the oxygen monitor cell or the sensor cell is a minute current on the order of μA and, hence, the detection accuracy degrades even if a very small leakage current flows thereinto.
Furthermore, as a gas sensing element to be built in a gas sensor for combustion control of a vehicle engine and other applications, there have been known many elements such as an element disclosed in Japanese Patent No. 2885336.
A description will be given hereinbelow of one example of a gas sensing element for the engine combustion control or the like.
That is, as FIG. 27 shows, the gas sensing element, generally designated at reference numeral 509, is made up of a heater substrate 515, a heat generator 561 formed on the heater substrate 515 and made to generate heat when energized, a heater 506 including a heater lead (not shown) electrically connected to the heat generator 561, a spacer 514 for a reference gas chamber 5140 into which a reference gas is introduced, a pump cell solid electrolyte plate 513 for formation of a pump cell 502, a spacer 512 for a measured gas chamber 5122 into which a measured gas is introduced from the external, another slid electrolyte plate 511 for formation of a sensor cell 504 and a monitor cell 503, and a spacer 516 for another reference gas chamber 5160, with these components being built up (piled up) into a multilayer construction.
Also in this construction, the sensor cell 504 is for measuring a specified gas concentration such as NOx concentration of a measured gas and the monitor cell 503 is for monitoring an oxygen concentration of the measured gas. Moreover, as shown in FIGS. 28A and 28B, pump leads 5211 and 5221 of the pump cell 502 are provided at central portions in width directions of the pump cell solid electrolyte plate 513.
One of electrodes constituting the sensor cell 504 is placed in opposed relation to the measured gas chamber 5122 and a specified gas in the interior of the measured gas chamber 5122 is decomposed on an electrode surface so that the concentration of the specified gas is measured on the basis of a current produced by the generated oxygen ions. Likewise, the monitor cell 503 has an electrode confronting the measured gas chamber 5122 to ionize oxygen on a surface of the electrode for measuring an oxygen concentration utilizing a current of the generated oxygen ions.
However, a current flowing through the monitor cell 503 and the sensor cell 504 is very weak, usually below 10 μA. On the other hand, a current flowing through the heat generator 561 or the heater lead of the heater 506 is as extremely large as 10A or less.
In most gas sensing elements, an insulating material (in the construction shown in FIG. 27, the spacer 514 or 512 can be made of an insulating material) or the like is interposed between the heater 506 and the electrodes of the sensor cell 504 or the monitor cell 503 to increase the electrical resistance of an electrical path formed between the heater 506 and the sensor cell 504 or the monitor cell 503. However, difficulty is experienced in reducing the leakage current to zero between the heater 506 and the sensor cell 504 or the monitor cell 503.
In addition, since the oxygen ionic current flowing through the sensor cell 504 or the monitor cell 503 is very weak as mentioned above, the effect of a small leakage current is not ignorable, and the leakage current causes the deterioration of measurement accuracy of the gas sensing element.